About half of the gasoline used in the United States is obtained by fluid catalytic cracking of petroleum. A fluid catalytic cracking unit consists of a cracking reactor, a catalyst regenerator and a fractionater. The catalyst employed is generally finely divided silica-alumina catalyst to which small quantities of finely divided large-pore zeolites are added to increase activity. The catalyst particles range in size from about 20 to 200 microns in diameter. The cracking reactor contains a bed of hot catalyst. Hot heavy petroleum fractions are passed into the catalyst bed which is maintained in a fluidized condition by passage of the feed and the cracked products thru it.
Catalyst in the reactor loses activity as a result of coke deposition on its surface during the cracking reaction. Catalyst is continuously withdrawn from the bottom of the cracking reactor and passed to the regenerator. In the regenerator the coked catalyst is contacted with hot air which burns coke from the catalyst surface and heats the catalyst to cracking temperature. Hot regenerated catalyst is withdrawn from the regenerator and returned to the cracking reactor together with additional petroleum to be cracked. Cyclone separators are disposed in the top of the reactor and separate most of the suspended catalyst particles from the cracked product which is removed from the top of the reactor and passed into the fractionating unit where light gases, gasoline, heating oil and heavy fuel oil fractions are separated and withdrawn as products. A small quantity of refractory hydrocarbons accumulates in the bottom of the fractionator. This refractory hydrocarbon material, which is commonly called slurry oil, contains a very small quantity of very finely divided (ca. 20 micron and smaller) catalyst particles which are not removed from the cracked products by the cyclone separators.
Slurry oil containing a very small fraction of a percent of catalyst fines is withdrawn from the bottom of the fractionater and passed into a very large tank where the catalyst fines settle. Periodically slurry oil is decanted from the tanks. Over a long period, usually three to five years, the tank is filled and may hold from three to eight thousand tons of slurry oil-catalyst fines mixture. The mixture is about 50% by weight of oil and 50% by weight of catalyst fines overall but concentration of fines is greatest near the tank bottom.
When the tank is filled it is opened and the oil-fines mixture is removed for disposal. Disposal is generally by incineration or by burial in a land fill area. Both methods of disposal have serious disadvantages. Incineration is expensive and wasteful, burial of the oil-fines mixture results in soil pollution at the dumpsites and contamination of subterranean water with polynuclear aromatic hydrocarbons contained in the oil which may be carcinogenic.
The means of disposing of slurry oil-catalyst fines mixtures must be improved.